In the area of digital printing (the term "printing" is used to encompass both printing and displaying throughout), gray level has been achieved in a number of different manners. The representation of the intensity, i.e., the gray level, of a color by binary displays and printers has been the object of a variety of algorithms. Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images contain an apparent continuum of gray levels. As an approximation to continuous tone images, pictorial imagery has been represented via binary halftone technologies. In order to record or display a halftone image with a scanning system, one picture element of the recording or display surface consists of a j.times.k matrix of sub-elements where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank, in other words, by suitably distributing the printed marks.
Halftone image processing algorithms are evaluated in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency rendition (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another method of producing gray levels is provided by gray level printing. In such a method, each pixel has the capability to render several different dot sizes. The dot size for a pixel is a function of the exposure time provided an LED element corresponding to that pixel. The longer the exposure time, the more toner is attracted to that particular pixel. See, for example, U.S. Pat. 4,680,645 for a method of rendering gray scale images with variable dot sizes.
There are two major concerns in rendering a continuous tone image for printing: (1) the resolution of image details, and (2) the reproduction of gray scales. In a binary halftone representation scheme, these two fundamental factors compete with each other. The more gray levels that are rendered, the larger is the halftone cell. Consequently, coarse halftone line screens are provided, with the attendant poor image appearance. Hence, a compromise is made in rendering between the selection of line resolution and gray scales in binary halftone printing. However, in gray level halftone printing, one can satisfy both resolution and gray level requirements. In gray level printing, the same number of addressable dots are present, and there is a choice of dot sizes from one dot-size of 1 bit/pixel to 16 different dot-sizes of 4 bit/pixel. An image could then be rendered with 133 line screens and 128 gray scales of higher quality image. Although providing higher image quality with respect to line resolution and tonal scales, gray level halftoning presents its own dot rendering issues.
It is desirable to provide an exposure time for the pixels such that there is an equal lightness change between steps in gray level halftone design. An example of a lightness vs. exposure time curve is shown in FIG. 9. This exposure curve is that obtained for continuous tone, which is then normally used for halftone. Such a curve works well for continuous tone output and gray level error diffusion. However, these methods have granularity limitations when the toner particles are relatively large (e.g. 12 microns volume diameter).
In gray level printing the exposure times are associated with different dot sizes. For example, with 3 bits/pixel, there can be 7 different exposure times, and 7 different dot sizes. A problem selecting the 7 exposure times from the continuous tone curve (which gives equal lightness change) can be recognized from FIG. 10, which shows lightness of the overall gray level of a halftone vs. a step number of the halftone, for a mixed dot type halftone cell described later. As can be readily seen, there is a non-even lightness jump in some of the steps. If the unevenness of this lightness jump is too great, noticeable density contouring in the output print will occur.
The reason behind this lightness jump comes from the fact that pixels within a cell grow differently depending on their surroundings, so that the exposure time for a continuous type of system (when all surrounding pixels are on) cannot be simply selected and used for every pixel in a halftone cell in a mixed dot type cell.
There is thus a need for a method and apparatus that reduces contouring by classifying pixels within a cell and modifying the exposure of the pixels according to this classification.